arc: cpu_idle: remove sleep workround for nsim_hs_smp

In old version nSIM, when cpu is sleeping, no response to
inter-processor interrupt although it's pending and interrupts
are enabled(SNPS JIRA issue P10019563-41294). Now this has
been fixed in nSIM version (2020.09), so we can safely remove it.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>